I did a few more iperf3 test (iperf3 -c ip-address -P5 -b 1000M -u) and now i believe the reported bitrates on the router are wrong.
I get a rx bitrate 800 - 900 mbit/s and tx bitrate around 500mbit/s (on the client).
I created an issue for the ath10k_ct driver /firmware: https://github.com/greearb/ath10k-ct/issues/150